[multipathtcp] [Iccrg] Re: Please review "Coupled Congestion Control for Multipath Transport Protocols"

[Thomas Dreibholz]

Dear all,

see my comments inline.


On Dienstag 01 Februar 2011, Costin Raiciu wrote:
> Hi Michael,
> 
> Sorry for the late reply - we haven't slept much lately because of the
> Sigcomm deadline.
> 
> When aiming for the same throughput as tcp on the best path, we equalize
> rates in bytes per second.
> 
> I have analyzed the formula in the draft again, and it is wrong for that
> context. The draft keeps the cwnd in bytes, and the correct formula in
> that case completely excludes MSS from the calculation of alpha. This is
> also the formula derived in the document you mention.
> 
> The formula for stacks that keep cwnd in packets needs to include MSS,
> hence the term cwnd_i *mss_i.

I do not understand why you use cwnd_i * mss_i here. If cwnd_i is given in 
bytes, then cwnd_i / mss_i gives the cwnd in MSS.

I can reconstruct the train of thought for the formula for alpha from 
"Practical Congestion Control for Multipath Transport Protocols". It is given 
in MSS instead of bytes.

To reconstruct the formula of the I-D, I have
- replaced  \hat{w}_r by (cwnd_i / mss_i) and
- used i instead of r as index variable.

The result is
alpha = tot_cwnd *
   ( max_i (sqrt(cwnd_i/mss_i) / rtt_i) /
     ((sum_i ((cwnd_i/mss_i) / rtt_i))^2) ).

To get rid of the square root function, it can be transformed to:
alpha = tot_cwnd *
   ( max_i ((cwnd_i / mss_i) / (rtt_i^2)) /
     ((sum_i ((cwnd_i/mss_i) / rtt_i))^2) ).

That is, that fomula in the I-D has to:
- replace "cwnd_i * mss_i" by "cwnd_i / mss_i" and
- "mss_i^2" by "mss_i".


Also, I am wondering why the fraction term is multiplied by tot_cwnd to 
compute alpha. The only place alpha is used is to compute the increase:

alpha*bytes_acked*mss_i/tot_cwnd

Since (tot_cwnd/tot_cwnd)=1, tot_cwnd may be simply cancelled out.


Best regards
-- 
=======================================================================
 Dr. Thomas Dreibholz

 University of Duisburg-Essen,                   Room ES210
 Inst. for Experimental Mathematics              Ellernstraße 29
 Computer Networking Technology Group            D-45326 Essen/Germany
-----------------------------------------------------------------------
 E-Mail:     dreibh at iem.uni-due.de
 Homepage:   http://www.iem.uni-due.de/~dreibh
=======================================================================
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 198 bytes
Desc: This is a digitally signed message part.
Url : http://oakham.cs.ucl.ac.uk/pipermail/iccrg/attachments/20110223/a88eda21/attachment.bin